A biological system is a complex network of biologically relevant entities. As biological organization spans several scales, examples of biological systems are populations of organisms, or on the organ- and tissue scale in mammals and other animals, the circulatory system, the respiratory system, the nervous system, etc.
On the micro- to the nanoscopic scale, examples of biological systems are cells, organelles, macromolecular complexes and regulatory pathways.
A biological system is not to be confused with a living system, which is commonly referred to as life. For further information see e.g. definition of life or synthetic biology.
Organ and tissue systems
These specific systems are widely studied in Human anatomy. "Human" systems are also present in many other animals.
- Circulatory system: pumping and channeling blood to and from the body and lungs with heart, blood and blood vessels.
- Integumentary system: skin, hair, fat, and nails.
- Skeletal system: structural support and protection with bones, cartilage, ligaments and tendons.
- Reproductive system: the sex organs, such as ovaries, fallopian tubes, uterus, vagina, mammary glands, testes, vas deferens, seminal vesicles and prostate
- Digestive system: digestion and processing food with salivary glands, esophagus, stomach, liver, gallbladder, pancreas, intestines, rectum and anus.
- Urinary system: kidneys, ureters, bladder and urethra involved in fluid balance, electrolyte balance and excretion of urine.
- Respiratory system: the organs used for breathing, the pharynx, larynx, bronchi, lungs and diaphragm.
- Endocrine system: communication within the body using hormones made by endocrine glands such as the hypothalamus, pituitary gland, pineal body or pineal gland, thyroid, parathyroid and adrenals, i.e., adrenal glands.
- Lymphatic system: structures involved in the transfer of lymph between tissues and the blood stream; includes the lymph and the nodes and vessels. The lymphatic system includes functions including immune responses and development of antibodies.
- Muscular system: allows for manipulation of the environment, provides locomotion, maintains posture, and produces heat. Includes skeletal muscles, smooth muscles and cardiac muscle.
- Nervous system: collecting, transferring and processing information with brain, spinal cord and peripheral nervous system.
- Immune system: protects the organism from foreign bodies